Shadowing Practice: Back to Back HVDC Modular Multilevel Converter for Transient Voltage Part 1 - Learn English Speaking with Video
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This is Jaan Girbadah Sumrur, Assistant Professor at Electrical Engineering Department, Sakhar IBE University.
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In our today's video we will discuss one of the research paper which is titled
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Back -to -Back HVDC Modular Multi -Level Convertor for Transient Voltage Dib Mitigation in Passive Networks.
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In my previous video I explained this research paper, Design and Control of Modular Multi -Level Convertor for Voltage Dib Mitigation.
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Now this work is done in 2022 and this work is totally inspired
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and this work totally follows basically this work which was done in 2018.
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2018. Alright.
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This was a thesis performed by a student of Bachelors.
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And he worked in CERN, European Organization for Nuclear Research for one year.
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Okay.
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For nuclear research laboratory like CERN, where the exceptionally high power quality conditioners are required,
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So there is a dynamic load, high power quality conditioners are used in CERN.
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The immunity against transient voltage dip is a vital importance.
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So if there comes the problem of voltage dip on the grid side, this is very important to mitigate that voltage dip.
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So the problem statement was to overcome the issue of voltage dip coming on the grid side that was affecting their load.
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By using the traditional HVDC MMC back -to -back configuration with properly dimensioned submodule capacitor, voltage diffs can be mitigated instantaneously.
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So they performed the steady on MMC back -to -back configuration to mitigate that issue of voltage dip,
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improving power quality of the network.
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The diminishing of the capacitor is shown as one of the key elements of this paper, yes,
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because the fault right through technique applied in this paper is
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by using inherent energy storage capacity of the capacitors in the MMV.
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That energy in capacitors is basically utilized to compensate for the missing energy of the voltage dip on the grid side.
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Firstly, the actual voltage dip statistics recorded at Sun are presented
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and described than the traditional capacitor submodule dimension method present compared to proposed energy design rule.
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To validate the energy dimension mitigation perform a detailed Simulink model of 220 MVHVDC modulo multilabular consider is described
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and the main results are presented.
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So here they use MATLAB Simulink.
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For this study, the paper concludes with the main conclusion of this study and the future works.
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Today's introduction, today several stable solutions.
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There are several solutions available for voltage mitigation.
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If for smaller load, UPS can be a good solution to get uninterrupted supply.
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In case of dip, UPS can work better.
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But the limitation here is the size of energy storage, which is preferable for battery storage system, and for small systems this can work.
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But for higher rating, the solution of UPS is not interesting, then researchers move towards DVR.
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But again, DVR is not the optimum solution.
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This paper suggests that the optimum solution is to, if you can afford, then the back -to -back HPDC -MMC can be a good
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solution for mitigating voltage -up issues to deliver constant load power supply to dynamic loads.
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The idea of using HVDC system completely based on VAC converters to supply a high power demand
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with high quality was analyzed in SEVEN where certain operation margin allowed to take over one converter from Nether and Kesa fault.
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If no operation margin is kept, all converters are running at one per unit, then another strategy is to implement the mitigate,
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has to be implemented to mitigate transient voltage dip.
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So if all the converters running at their full capacity one per unit,
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then there need to be some fault right through strategy applied to mitigate the voltage dip,
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voltage sake issue occurring at the gate side.
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One of the solutions used by researchers in previous days was
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by allowing higher currents to flow during the time of transient voltage dip.
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a compensation effect could be attained.
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So if there was a voltage dip, so to compensate that missing energy of the voltage dip or voltage sake,
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higher current, probably two times more than normal current, was taken from the grid to mitigate that, to compensate that missing energy.
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But this is not a good solution, not recommended.
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This solution was investigated in research paper 8, concluding that by allowing two times the nominal current,
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the HVDCVAC solution can protect our network from voltage dip.
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So they observed that if you, during the voltage dip, if you take two times more current than the normal current,
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the voltage dip issue was being solved out.
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Regarding the specific voltage dip mitigation for HVDC links, MMC topology results the most interesting.
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For these convertors, the electric energy is stored in the capacitor
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which they need in any case for the normal operation can be partially used to support the voltage during transient voltage dips.
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So as you know that in MMC, in case of half -built cell or full -built cell,
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both topologies or every topology has a cell, a capacitor cell.
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And one of the key characteristics of MMC is inherent energy storage capability.
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So that energy can be utilized to compensate for the missing energy, caused missing energy due to the voltage dip or voltage.
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So this capacitor energy can be potentially utilized in this application.
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In principle, it could be expected that they provide the fast response thanks to the fast release of energy in the capacitor.
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The energy in the capacitor can be released fast.
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And fast enough that your DC link does not go to the undervoltage.
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And the load should not experience even slight disturbance.
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So you should release the capacitor energy very quickly, and this is possible to release it,
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so that load may not get disturbance.
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This voltage -dependent grid side should not affect the load.
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This is the objective.
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To explore the potential application of HVDC MMC for voltage -dependent mitigation purpose,
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an essential question is how much of capacitor energy is needed for good performance.
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This is the question.
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So now the question is you need to know how much is missing energy due to the voltage dip
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and how much extra energy you need to store in the capacitor to mitigate
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that dip issue or to compensate the missing energy.
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How much extra energy do you need to store it?
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In this sense the first decision to be made is to choice between the transient voltage voltage dip mitigation strategies,
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and increasing line current strategy and energy storage based solution.
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So there are two approaches.
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One approach to mitigate that voltage dip strategy is increasing line current strategy, which is not good, because if you are using increasing line current strategy,
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then the higher currents two times more than normal current will flow.
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In that case, the converter size will be overrated, converter station will be more costly, more having more footprint weight.
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So this is not recommended.
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The recommended approach here in this paper, the energy storage -based solution.
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But now researchers are working that even the best choices,
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the controller design should be such effective that neither we should prefer this line current strategy or energy -based storage system.
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But there should be automatic switching.
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The controller should be fast enough and robust enough to mitigate this issue.
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So researchers are also working in this area.
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Anyhow, in this paper the technique suggested is energy storage based solution.
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We will discuss energy storage based solution.
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The first storage strategy is allowing the rectifier to draw increased current during the voltage dips such
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that the DC bus transfers nominal power at all times.
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With the inverter synthesizing AC voltage for the load, applying this solution with MMC means
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that semi -calorie switching devices in sub -module capacitor must be
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highly overrated compared to record rating for normal operating condition in the rectifier, making the solution less elegant and interesting.
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Therefore, increasing line current is not good enough because it will increase the overall converter station size cost.
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The second study which is one of the one
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which is the one proposed in this paper is to design the MMC sub -module
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capacitors according to dedicated energy calculation presented in the energy calculation chapter.
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The rectifier shall be designed to draw nominal current from the gate during a transient voltage dip,
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extracting the maximum allowed power to optimize the needed storage for dip mitigation.
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The advantage is that both rectifier inverter will be dimensioned for normal operation condition
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and by using proposed design method for the overall system design optimum with it.
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So this is a better approach to use that capacitor energy storage property to mitigate this dip Thank you.
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In general, the MMC shows very high efficiency and provides several power quality conditioning features to passive networks, such as voltage dip mitigation, reactive power compensation.
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So reactive power compensation is also possible with MMC -based HVDC system.
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A passive network in this synopsis in the paper is defined as a network with a short circuit ratio of zero,
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which means no generation of energy.
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Okay.
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Behind acting as a dip mitigation,
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the inverter can provide reactive power support for the passive load network in two coordinates and even supporting pulsating dynamic loads.
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So this is an additional advantage
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that reactive power support of the load is also possible with reactive power controller on the inverter side.
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In the case of particle excise, this could be a very beneficial feature for it could replace a static VR compensator,
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so no need of a static VR compensator in this case.
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If you have back -to -back HVDC MMC system, the rectifier has the ability to support the grid with reactive power injection during phase
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and can allow power oscillation dipping and grid destabilization.
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Lastly, the system features steady state control allowing high precision of voltage and frequency regulation for the passive network.
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So even during this voltage dip, there will be no effect on the load.
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The load will be getting the constant power supply.
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There will be no disturbance on the load side.
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This paper is in general analysis of the potential of transient voltage mitigation of an MMC -based HVDs back -to -back link.
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An important part is the dimensioning of the capacitors.
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This is very important.
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Analyze an energy dimensioning chapter, which highlights also the correlation between storage size and dip mitigation performance.
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So certainly the storage size capacitor dimensioning will depend upon what is the dip severity, how much dip is happening,
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for how many seconds, how much amplitude of dip is there, depending upon the dip severity, amplitude of the dip, time of the dip,
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capacitor dimensioning will be performed, energy storage size will be performed.
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And this is performed in detail in the thesis work of this same author.
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You can also refer to that thesis of this author.
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After the main simulation model is present, including also the control circuit, the conclusion shows the main result concerning dip mitigation performance.
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OK, the energy dimensioning, which is very important because it uses capacitor energy storage property.
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So energy dimensioning of capacitor becomes important.
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The target of this paper is to show the potential of MMC -based HPD -link system for transient voltage dip mitigation
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and give insight in energy storage calculation for the modules.
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This will mean a new proposed design rule for MMC converter when transient voltage dip mitigation is required.
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The traditional way for storage design in the module
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which is done in the industry is to design the energy storage of MMC converter around 30 to 40 kilojoule per mva.
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So converter registration, traditional way of storage design is that range is 30 to 40 kilojoule per mva per converter registration.
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For a break to break system this results in total system energy because one is a rectifier other is inverter,
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so total system energy is of 60 to 80 kJ per mVA.
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But with the object or dip mitigation,
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a new design for the value of stored energy per mVA has to be derived because this is for the normal case.
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But if you want to mitigate the voltage dip without taking increased line current strategies then this need to be recalculated.
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The first step is to analyze the discharge capabilities of submodule capacitors.
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If the modular multilevel converter is assumed to operate with a DC voltage of 1 per unit,
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so the requirement is that you need to maintain DC voltage 1 per unit, means whatever the desired reference voltage is, that should be tracked.
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In per unit it should be 1 per unit.
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At modulation index of 0 .8 in a steady state.
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So in a steady state, modulation index value uses, you prefer 0 .8.
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The inverter is limited to minimum DC voltage input of 0 .8 per unit.
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That means DC link should not fall below 0 .8 per unit.
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If it falls below the 0 .8, if it becomes 0 .4, 0 .5 per unit, then the inverter has to operate in our modulation,
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which is not required, which is a big problem.
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Our modulation is an issue, and if there comes opposite problem, not the undervoltage, but our voltage in the DC link,
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that is also an issue that will burn the DC cable, our MMC converter station and HVDC can collapse.
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So both under -voltage and over -voltage in the DC link are not desired.
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The total energy stored for both inverter and rectifier depends on DC voltage of the bus.
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The following is the fundamental equation for energy in the capacitor.
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This is a well -known equation of energy stored in the capacitor 1 over 2 Cv square.
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With this in mind, the total energy store must be high
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that after mitigating a dip the DC voltage should not be below 0 .8 per unit based on the previous equation. So, we...
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We have to make sure that capacitor discharges the energy fast enough so that DC voltage should not fall below 0 .8.
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This means that capacitor only allowed to discharge up to 36 % of their initial steady -state charge.
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This result concludes the total energy of the capacitor should be damaged 2 .77 times
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the missing energy for the inverter operating with modulation index of 0 .8 means if the missing energy,
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whatever the missing energy is divided by 0 .36
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this means the missing energy stored in the capacitor should be 2 .77 times more than the missing energy.
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Calculate the missing energy due to the voltage dip
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and then you should store 2 .77 times more as compared to E missing, multiplied by E missing.
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So let's say if E missing is 0 .3 then E stored should be 2 .77 into 0 .3.
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Second step is to find the missing energy of the system caused by the dip.
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So a statistical analyzer of all mile voltage dip in a highly sensitive distribution present in figure 1.
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This shows that this picture shows voltage dip magnitude percentage and for the time,
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how much time the dip remains.
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And accordingly, depending upon this curve,
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we will come to know that how much energy need to be stored in a back -to -back HVDC MMC system.
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Okay.
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In a function called dip severity is introduced, dip severity, this function is a dip magnitude multiplied by dip duration.
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If the dip duration, let us say if it is for 75 millisecond
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and the dip magnitude is 50 50 % multiplied by 0 .75 ms will be the dip severity. So
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if you multiply both dip duration with the dip magnitude
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you will get dip severity where dips are assumed by 3 phase dips.
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The calculation operates in per unit system with 1 second days per unit in time.
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This makes power and energy base equal.
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Figure 2a shows histogram of dip severity for all required dips and log logistic distribution function is fitted to data.
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As it can be observed the majority of registered voltage dips are included in a window of 50 % at 150 milliseconds.
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So figure 2a, the data was collected, it was observed
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that the most repeated pattern of the dip severity was 50 % dip for 150 milliseconds in the CERN network.
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Figure 2b shows distribution density function rate and the cumulative function
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essentially shows how to dimension the stored energy of the converter as a function of confidence.
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With 99 % being the chosen value for the study,
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more trivially explained that 99 % confidence translates to 99 % of all the voltage diff will be mitigated.
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So in this research paper,
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Here the capacitor dimensioning or energy dimensioning is made not 60 to 80 kJ per mEV
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because this is for the normal case but for voltage dip this needs to be recalculated.
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So it is designed for such a value so that 99 % of dips are successfully mitigated.
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The fact that rectifier will always draw a nominal current from the grid means
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that dip severity in the faked missing energy caused by the dip.
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So E missing is current dip, change in current dip multiplied by change in V dip multiplied by time in T dip,
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how much time dip lasts.
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By graphical solution of our 99 % confidence figure to be the missing energy is seen as 0 .0375 per unit.
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So if you see here,
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the missing energy due to the dip in the sun network with
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that particular load was missing energy in most of the time, 99 % of the time was 0 .0375 per unit.
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This was the missing energy due to the dip.
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So if this is a missing energy now, how much energy should be stored in the capacitors?
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it should be e stored should be 2 .77 times this missing energy
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so multiply 2 .77 with this value you will get
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that total stored energy should be 0 .104 per unit of the system for the system
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Assuming a system of 1 mV rating this means that the reactive inverter together must have an energy of
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104 .0 .104 means it should have energy of 104 kJ per mVa.
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Normally it is 60 to 80 kJ per mVa but for the mitigation of voltage dip and for
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compensating the missing energy
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and to make sure the 99 percent of the voltage dip
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severities are successfully mitigated the system should be now having energy storage of 104 kJ per mVA.
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Now this is the disadvantage
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that overall energy diminishing is changing overall capacitor sizing will be different
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so this will make this converter station little bit cost a little bit expensive as compared to normal.
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So this approach has the disadvantage in terms of cost.
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Comparing this result with the commonly used 60 to 80 kJ per meter shows by only a slight increase of stored energy.
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So there is a slight increase, normally it is 80 kJ per mva
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but to mitigate the voltage it should be the system should be 104 kJ per mva.
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99 % all volt dips will be mitigated if the system is off, 104 kilojoule per MVA.
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Energy storage is this one.
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OK.
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So in the next part, we will discuss simulation model and the results.
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Thank you.
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Vocabulary and speaking notes for this lesson
This speaking lesson is built on the video “Back to Back HVDC Modular Multilevel Converter for Transient Voltage Part 1”. The speaker keeps coming back to these words: dip, energy, voltage, capacitor, missing. This video has 230 sentences and 3097 words to shadow. The speech runs for 21:27. The speaker talks at a steady 144 words per minute, a comfortable pace for shadowing. Only 77% of the words are among the 3,000 most common in English, so the vocabulary is demanding.
Key vocabulary in this video
15 words from the video worth learning, with pronunciation and meaning:
| Word | Pronunciation | Meaning |
|---|---|---|
| voltage noun | /ˈvəʊltɪd͡ʒ/ | The difference in electrostatic potential between two points in space, especially between live and neutral conductors or the earth. |
| mitigate verb | /ˈmɪt.ɪ.ɡeɪt/ | To reduce, lessen, or decrease and thereby to make less severe or easier to bear. |
| dimension noun | /daɪˈmɛn.ʃən/ | A single aspect of a given thing. |
| severity noun | /səˈvɛɹ.ə.ti/ | The state of being severe. |
| dip verb | /dɪp/ | To lower into a liquid. |
| grid noun | /ɡɹɪd/ | A rectangular array of squares or rectangles of equal size, such as in a crossword puzzle. |
| reactive adjective | /ɹiːˈæktɪv/ | That reacts or responds to a stimulus. |
| compensate verb | /ˈkɑm.pənˌseɪt/ | To do (something good) after (something bad) happens. |
| steady adjective | /ˈstɪdi/ | Firm in standing or position; not tottering or shaking; fixed; firm. |
| passive adjective | /ˈpæs.ɪv/ | Being subjected to an action without producing a reaction. |
| magnitude noun | /ˈmæɡnɪtjuːd/ | The absolute or relative size, extent or importance of something. |
| researcher noun | /ˈɹiˌsɝ.t͡ʃɚ/ | One who researches. |
| module noun | /ˈmɑd͡ʒul/ | A self-contained component of a system, often interchangeable, which has a well-defined interface to the other components. |
| calculation noun | /ˌkælkjuˈleɪʃn̩/ | The act or process of calculating. |
| nominal adjective | /ˈnɒm.ɪ.nl̩/ | Of, resembling, relating to, or consisting of a name or names. |
Phrasal verbs you will hear
| Word | Meaning |
|---|---|
| take over verb | To assume control of something, such as a business or enterprise, and sometimes by force. |
Grammar in this video
The structures the speaker uses most, with the exact words from the video:
| Structure | In the video |
|---|---|
| Passive voice be + past participle — the focus is on what happens, not who does it | is titled · are required · can be mitigated |
| Relative clauses who / which + clause — extra information about a person or thing | paper which is · work which was · storage, which is |
Pronunciation to watch
- The “th” sounds: thesis /ˈθisɪs/, synthesize /ˈsɪnθəsaɪz/
- The “sh” and “zh” sounds: dimension /daɪˈmɛn.ʃən/, calculation /ˌkælkjuˈleɪʃn̩/, compensation /ˌkɑm.pənˈseɪ.ʃən/, duration /dʒʊˈɹeɪ.ʃən/, equation /ɪˈkweɪ.ʒən/
- Long words — get the stress right: severity /səˈvɛɹ.ə.ti/, compensation /ˌkɑm.pənˈseɪ.ʃən/, successfully /səkˈsɛs.fə.li/, registration /ˌɹɛd͡ʒ.ɪˈstɹeɪ.ʃən/, capability /ˌkeɪ.pəˈbɪl.ɪ.ti/
How to practise with this video
- Listen to the whole video once without speaking and note the words you do not know.
- Shadow it sentence by sentence at normal speed, repeating each one until your rhythm matches the speaker.
- Record yourself and compare with the original, paying attention to words like voltage, mitigate, dimension.
What is the Shadowing Technique?
Shadowing is a science-backed language learning technique originally developed for professional interpreter training and popularized by polyglot Dr. Alexander Arguelles. The method is simple but powerful: you listen to native English audio and immediately repeat it out loud — like a shadow following the speaker with just a 1–2 second delay. Unlike passive listening or grammar drills, shadowing forces your brain and mouth muscles to simultaneously process and reproduce real speech patterns. Research shows it significantly improves pronunciation accuracy, intonation, rhythm, connected speech, listening comprehension, and speaking fluency — making it one of the most effective methods for IELTS Speaking preparation and real-world English communication.